In addition to horn, rod and parabolic antennas, use is also made of planar antennas in the technology of measuring filling levels. Refinements of planar antennas are described, for example, in the book “Einführung in die Theorie und Technik planarer Mikrowellenantennen in Mikrostreifenleitungs-technik” [“Introduction to the Theory and Technology of Planar Microwave Antennas in Microstrip Line Technology”], by Gregor Gronau, Verlagsbuchhandlung Nellissen-Wolff or in the journal article “Impedance of a radiating slot in the ground plane of a microstrip line”, IEEE Trans. Antennas Propagat., Vol. AP-30, 922–926, May 1982.
A planar antenna usually comprises a dielectric substrate on one side of which the antenna structure is provided, and on the other side of which a conductive coating is provided. An asymmetric stripline is the basis of the most widespread planar antenna structure.
Moreover, it is usually possible to provide cutouts in the conductive layer, in which case supply lines to the cutouts are then to be provided on the other side of the dielectric substrate. These cutouts are, for example, configured and arranged in such a way that the antenna preferably radiates only electromagnetic waves of a desired mode. A planar antenna for use in an explosive environment is disclosed in Published European Patent Application No. EP 1 083 413 A1. This known solution can, of course, also be used in combination with the invention described in the present application. This known solution is therefore expressly to be ascribed to the disclosure content of the present application.
Usually (Gaussian) delta-shaped pulses, also frequency-modulated continuous signals are used as measuring signals. Both types of measuring signals are relatively broadband. For example, the pulsed measuring signals have a bandwidth of a few hundred megahertz. The planar antennas which have become known are capable, however, only of radiating the measuring signals in a quasi-punctiform frequency range at the resonant frequency in a virtually undamped fashion. As can be seen from FIG. 2, all frequency components of the measuring signal which are outside the “resonant frequency” of the cutouts are more or less reflected.